about high resistance

[faili]

I used NI 4071 to measure a high resistance. The resistance is 78.6MΩ.

When I use 4071 directly to measure it(2-wire), the result is 78.58MΩ.

The measurement is ok. But wnen I connect the resistance to NI Switch 2575,

and measure it.The result become smaller, about 76.4MΩ。

It is strange, and how to deal with the problem?  Thanks

[gwd]

Something (wiring, the switch board, etc.) is introducing additional parallel resistance. Look at the specs on the switch board. If the leakage specs are excellent (> 1e9 ohms) then you can turn your attention to the wiring.